The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological progress in IC manufacture has produced several generations of ICs, and each generation fabricates smaller and more complex circuits than the previous generation. Currently, the semiconductor industry has progressed into nanometer technology nodes for higher device density and better electrical performance, and a variety of challenges from fabrication and design have led semiconductor technologies to three dimensional designs, such as fin-like field effect transistors (finFETs). A typical finFET is fabricated with a thin “fin” extending on a substrate. The channel of the finFET is formed in the fin. In addition, a gate is formed to traverse the fin, and therefore a tri-gate structure is fabricated. It is beneficial to have a gate on three sides of the channel that allows the gate to control the channel from several sides. The finFET devices further include strained source/drain features to enhance carrier mobility and improve device performance. However, as device scaling down continues, conventional techniques have not been entirely satisfactory in all respects.